1. Field of the Invention
The invention relates to the field of telecommunications and concerns a signaling terminal system for a digital communications network employing the No. 7 common channel signaling system defined by the CCITT (see Yellow Book, Fascicle VI.6--Recommendations Q.701-Q.741).
2. Description of the Prior Art
Communication systems involve the transfer of information between a subscriber and an exchange or between two exchanges. This is effected by the interchange of so-called signaling signals. If the signaling signals are transmitted on the same circuit as that used for communication, the signaling method is known as channel associated signaling. On the other hand, if the signaling signals corresponding to a group of circuits between two centers are combined together and transmitted on a separate channel, the signaling method is known as common channel signaling.
The CCITT No. 6 signaling system was the first signaling system of this kind to be specified, applied and tested. Having been originally conceived for analog applications, although subsequently modified for digital versions, the No. 6 system has limitations in a digital environment (bit rate, message length, etc.). So, to meet new requirements (digital networks), the CCITT drew up a new specification, the so-called No. 7 common channel signaling system, having a broad range of applications in dedicated and multiservice networks.
The transmission protocol employed is derived from the HDLC protocol and in this sense is related to the procedure defined in Recommendation X.25 relating to the DTE/DCE (data terminal equipment/data communication equipment) interface.
Systems using common channel signaling terminals to process protocols similar to that used in the No. 7 signaling system and providing for the connection of a number of channels are either hardwired systems (which multiplex the channels) or systems employing bit slice microprocessors. The resulting structures feature low levels of modularity and flexibility, and are difficult to use.
A general objective of the invention is to provide a signaling terminal system for the No. 7 signaling system which is flexible in use and modular in design.